3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs.
Furthermore, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), together referred to as High Speed Packet Access (HSPA), are mobile communication protocols that were developed to cope with higher data rates than original Wideband Code Division Multiple Access (WCDMA) protocols for UMTS were capable of.
The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB or eNB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. In UMTS, a Radio Network Controller (RNC) controls the NodeB, and is, among other things, in charge of management of radio resources in cells for which the RNC is responsible. The RNC and its corresponding NodeBs are called the Radio Network Subsystem (RNS). The RNC is in turn also connected to the Core Network (CN). In LTE, the eNodeB manages the radio resources in the cells, and is directly connected to the CN, as well as to neighboring eNodeBs via an X2 interface.
FIG. 1 illustrates an example deployment of a radio access network in an LTE system. An eNB 101a serves a UE 103 located within the RBS's geographical area of service or serving cell 105a. The eNB 101a is in this example connected via an X2 interface to a neighboring eNB 101b serving another cell 105b. 
Multi-Carrier or Carrier Aggregation Concept
The LTE Rel-10 specifications have been standardized, supporting Component Carrier (CC) bandwidths up to 20 MHz, which is the maximal LTE Rel-8 carrier bandwidth. An LTE Rel-10 operation wider than 20 MHz is possible and appears as a number of LTE CCs to an LTE Rel-10 terminal. The straightforward way to obtain bandwidths wider than 20 MHz is by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CCs, where the CCs have or at least have the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 2, where five 20 MHz CCs are aggregated totaling 100 MHz. In CA operation the UE is thus able to receive and/or transmit data from and to more than one cell. In other words, a CA capable UE can be configured to operate with more than one serving cell. A carrier of each serving cell is generally called a CC. The CC is thus an individual carrier in a multi-carrier system. A CA system may alternatively be called a multi-carrier system, a multi-cell operation system, multi-carrier operation system, or a multi-carrier transmission and/or reception system. CA is used for transmission of signaling and data in the uplink and downlink directions. One of the CCs is designated as the primary component carrier (PCC). A PCC may also be referred to as a primary carrier, an anchor carrier, a primary cell (PCell), or a primary serving cell (PSC). The remaining CCs are designated as secondary component carriers (SCC). An SCC may also be referred to as a secondary carrier, a supplementary carrier, a secondary cell (SCell), or a secondary serving cell (SSC).
Generally, the PCell carries the essential UE specific signaling and is the carrier where the UE performs radio link monitoring. The PCell exists in both uplink and downlink directions in CA. In case there is a single UL CC the PCell must be on that CC. The network may assign different PCells to different UEs operating in an area within radio coverage of the same sector or cell.
Multi-Carrier SCell Setup or Release Procedure
A multi-carrier SCell setup refers herein to a procedure which enables the network node to at least temporarily setup or release the use of an SCell, in downlink (DL) and/or uplink (UL) by the CA capable UE. Herein the SCell setup or release procedure or command can comprise one or more of the following:                Configuration of SCell(s) also known as SCell addition (setup)        De-configuration of SCell(s) also known as SCell release (release)        Activation of SCell(s) (setup)        Deactivation of SCell(s) (release)Configuration and De-Configuration of SCell        
The configuration procedure of an SCell, i.e. addition/release of SCell, is used by the serving radio network node, e.g., eNodeB in LTE or NodeB in HSPA, to configure a CA-capable UE with one or more SCells, e.g., with DL SCell, UL SCell or both. On the other hand, the de-configuration procedure is used by the serving radio network node or RBS (eNodeB or NodeB) to de-configure or remove one or more already configured SCells, e.g., DL SCell, UL SCell or both. The configuration or de-configuration procedure is also used to change the current multi-carrier configuration, e.g., for increasing or decreasing the number of SCells or for swapping the existing SCells with new ones. The configuration and de-configuration are done by the eNodeB and by RNC using Radio Resource Control (RRC) signaling in LTE and HSPA respectively.
Activation and Deactivation of Secondary Cells
The serving radio network node, e.g., eNodeB in LTE or NodeB in HSPA, can activate one or more deactivated SCells or deactivate one or more active SCells on the corresponding configured secondary carriers. The PCell is always activated. The configured SCells are initially deactivated upon addition and after a cell change, e.g., a handover. In HSPA the activation and deactivation command is sent by the NodeB via a High Speed-Shared Control Channel (HS-SCCH). In LTE the activation and deactivation command is sent by the eNodeB via a Media Access Control (MAC) control element (MAC-CE). The deactivation of SCell saves UE battery power.
In the existing solutions, SCell activation and deactivation delay requirements exist only for one SCell as explained below:                A. SCell activation delay: The delay within which the UE shall be able to activate the deactivated SCell depends upon the specified conditions and also on the number of CCs supported by the UE. Upon receiving SCell activation command in subframe n, the UE shall be capable to transmit a valid Channel State Information (CSI) report for the SCell being activated no later than in subframe n+24, provided certain pre-defined conditions are met for the SCell and the UE is configured with one SCell. Otherwise upon receiving the SCell activation command in subframe n, the UE shall be capable to transmit a valid CSI report for the SCell being activated no later than in subframe n+34 provided the SCell can be successfully detected on the first attempt and is configured with one SCell. The valid CSI is based on the UE measurement and corresponds to any pre-defined Channel Quality Indicator (CQI) value with the exception of CQI index=0 (meaning out of range). In case the UE is configured with two or more SCells then the activation delay can be longer than 24 subframes or 34 subframes.        B. SCell deactivation delay: Upon receiving SCell deactivation command or upon expiry of the sCellDeactivationTimer in subframe n, the UE shall accomplish the deactivation actions for the SCell being deactivated no later than in subframe n+8.Licensed-Assisted Access (LAA) to Unlicensed Spectrum Using LTE        
The unlicensed spectrum, which in, e.g., the 5-6 GHz range can be found between 5150 MHz and 5925 MHz, can be simultaneously used or shared by multiple different technologies, e.g., by LTE and Institute of Electrical and Electronics Engineers (IEEE) Wi-Fi. The LAA intends to allow LTE equipment to also operate in an unlicensed radio spectrum. Note that, the same LAA concept can be used in other spectrum too, such as in the 3.5 GHz range in North America. In LAA mode, devices connect in the licensed spectrum to a primary cell or PCell, and use CA to benefit from additional transmission capacity in the unlicensed spectrum via a secondary cell or SCell. Therefore, the UE can be configured with one or more SCells in the unlicensed spectrum.
Since the unlicensed spectrum must be shared with other wireless technologies (e.g., Wi-Fi, radar, Bluetooth, fixed satellite system), a so called Listen-Before-Talk (LBT) method needs to be applied. LBT involves sensing the medium for a pre-defined minimum amount of time to determine whether there is a transmission or not and thus whether the channel is busy or not, and backing off if the channel is busy. There will thus be no transmission if there already is a transmission on the channel. FIG. 3 illustrates LAA to unlicensed spectrum using LTE CA, with an UL and DL PCell operating on licensed spectrum and an SCell operating on unlicensed spectrum.
Listen-Before-Talk (LBT)
According to the LBT procedure the transmitter or transmitting node that wishes to transmit in unlicensed spectrum (e.g., the radio base station in case of DL or the UE or wireless device in case of UL) needs to listen on the carrier before it starts to transmit. If the medium is free the transmitter can transmit, while if the medium is busy, e.g., some other node is transmitting, the transmitting node cannot transmit and the transmitting node may try again at a later time. Therefore, the LBT procedure enables a Clear Channel Assessment (CCA) check before using the channel. Based on the CCA, if the channel is found to be clear then then LBT is considered to be successful. But if the channel is found to be occupied then the LBT is considered to be a failure also known as an LBT failure. The LBT failure requires the transmitting node not to transmit signals in the same and/or subsequent subframes. Exact subframes and also number of subframes where transmission is forbidden depends on the specific design of the LBT scheme.
Due to LBT, a transmission in an unlicensed band may be delayed until the medium or channel becomes free again. And in case there is no coordination between the transmitting nodes (which is often the case) the delay may appear randomly.
In the simplest form, LBT is performed periodically with a period equal to certain units of time. As an example, one unit of time may be one Transmission Time Interval (TTI), one timeslot, or one subframe. The duration of listening in LBT is typically in the order of a few to tens of μseconds. Typically, for LBT purpose, each LTE subframe is divided in two parts: in the first part, the listening takes place and the second part carries data if the channel is seen to be free. The listening occurs at the beginning of the current subframe and determines whether or not data transmission will continue in this subframe and a few next subframes. Hence, the data transmission in a subframe P until subframe P+n is determined by the outcome of listening during the beginning of subframe P. The number n depends on system design and/or on regulatory requirements.
Discovery Reference Signal
The Discovery Reference Signal (DRS) is any type of reference or pilot signal which is pre-defined or pre-configured at the UE. In LAA, DRS in the downlink may be used for enabling the UE to perform functions such as channel estimation, synchronization to a cell, Automatic Frequency Control (AFC), Automatic Gain Control (AGC), and radio measurements. Examples of radio measurements are cell search, Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ) measurements, positioning measurement, or CSI measurements. Examples of CSI measurements are CQI, Rank Indicator (RI), Pre-coding Matrix Indicator (PMI) measurements.
The transmissions of the DRS occur in DRS occasions. The DRS may comprise, e.g., the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), Cell Reference Signal (CRS), and CSI-Reference Signal (CSI-RS). The UE is configured with a Discovery Measurement Timing Configuration (DMTC) which is a time window within which the UE can receive the DRS. The DMTC provides a window with a duration (e.g., between 1-6 ms), also known as DRS occasion, occurring with a certain periodicity and timing within which the UE may expect to receive discovery signals or DRS. Examples of DRS occasion periodicity are 40, 80, or 160 ms.
Due to LBT, there will be some instances or occasions where the transmitting network node is unable to transmit DRS. Thus, DRS will not be transmitted in every DRS occasion. If LBT is applied to DRS transmissions, there will be some instances where the DRS is not able to be transmitted in a periodic manner as in the case of the Rel-12 DRS transmitted on a cell in licensed spectrum. The following two options may then be considered for DRS design for LAA, as described in 3GPP TR 36.889 version 13.0.0, 2015-06.                1. Subject to LBT, DRS is transmitted in a fixed time position within the configured DMTC;        2. Subject to LBT, DRS is allowed to be transmitted in at least one of different time positions within the configured DMTC        
The two alternatives above are shown in FIG. 4, referred to as Alt.1 and Alt.2 respectively.
Standalone Access of Unlicensed Spectrum Using LTE
There will also be LTE systems operating in unlicensed spectrum completely in a standalone manner. The difference between LAA and standalone LTE will be that there will not be any licensed carrier to be aggregated with the unlicensed carrier in standalone usage, while an unlicensed LTE is always aggregated with licensed carrier in LAA operations. Standalone operation means that the UL will also be allowed in unlicensed spectrum usage of LTE. Since there will not be any support from a licensed carrier, the standalone LTE system is responsible for all functionalities in unlicensed spectrum.
In a standalone operation, a UE may be capable of using a single unlicensed carrier, or may be capable of aggregating more than one unlicensed carriers. In the latter case, both PCell and SCell(s) will be in unlicensed spectrum.
LAA Operation in Dual Connectivity Mode
In LAA, the unlicensed carrier can also be aggregated with a licensed carrier in a dual connectivity manner. In Dual Connectivity (DC) mode, at least one CC in a Master eNodeB (MeNB) is termed as PCell and at least one CC in a Secondary eNodeB (SeNB) is termed as PSCell. PCell and PSCell are functionally similar nodes. However, activation/deactivation/configuration/deconfiguration of PSCell is controlled by a PCell. The connected nodes in DC operation are independent to each other, thus, all control signaling is done separately.
Problem Description
In e.g. LAA or standalone LTE access of unlicensed spectrum, one or more CCs may belong to an unlicensed frequency band (e.g., in the 5 GHz range). The unlicensed frequency band can be shared between multiple wireless devices of different operators. To allow fair sharing of spectrum, mechanisms such as LBT will be applied on CCs of the unlicensed band. When CA is applied for PCell and SCell(s), an SCell on any CC can be configured, de-configured, activated, or deactivated at the UE by the network node, e.g., the eNodeB. As stated above, due to LBT some of the DRS signals may not be transmitted by the network node, or may be delayed, as the channel or medium may be busy by other transmissions in the DRS occasion. If the SCell (or a standalone carrier) is operating in the unlicensed band the UE may not be able to fulfil the requirements (e.g., delays) associated with e.g. cell activation procedures due to the LBT procedure, since the UE uses DRS to achieve synchronization.